1. Field of the Invention
This invention relates to a photopolymerizable composition, and more specifically to a photopolymerizable composition suitable as a solder resist for producing a printed circuit board. Further, this invention relates to a method for forming a cured coated film using the above photopolymerizable composition.
2. Prior Art
In order to avoid deposition of a solder on areas other than the required area in soldering parts to a printed circuit board, etc., a solder resist has been applied so far for protecting the circuit on the surface of the printed circuit board.
The solder resist has been applied, in many cases, by printing the resist composition on a substrate by a silk screen printing method, or the like in accordance with a required pattern, and then heat-curing the composition or curing it by actinic radiation such as ultraviolet light. However, with a trend toward higher density of the printed circuit board owing to smaller size or higher accuracy of electric and electronic parts, a liquid phototype composition with higher patterning accuracy has been proposed.
For example, it has been proposed that a film with a necessary pattern drawn thereon is adhered to a resist film obtained by coating a liquid photopolymerizable solder resist on a printed circuit board by silk screen printing, roll coater coating or the like and drying it to touch by heating, etc. and is irradiated with actinic radiation such as ultraviolet light or the like to cure exposed areas. The unexposed portion is dissolved and removed with a developer to form a solder plating pattern and a circuit protective film (U.S. Pat. No. 4,786,579).
With a fine circuit pattern, not only improved solder heat resistance and chemical resistance but also excellent impact resistance has been lately required of the solder resist. To meet this requirement, we have made various investigations over modification of the known liquid resist resin with organopolysiloxane by blending or chemical bonding.
When using, however, the thus modified resin as a liquid resist, storage stability decreases owing to phase separation in case of the blending, and workability decreases by increase in viscosity because the molecular weight of the product is difficult to control in case of the chemical bonding. Further, the use of polysiloxane enhances the hydrophobic nature of the resin, decreasing developability.
Still further, the conventional resist uses an organic solvent or a dilute alkali aqueous solution as a developer. The use of the organic solvent as the developer poses risks of fire hazards, toxicity to human beings and other safety or sanitation problems. Meanwhile, a resist using a dilute alkali aqueous solution as a developer is inferior in performance to a heat curing-type or organic solvent developing-type resist.
In order to eliminate the defects of the conventional resist, the present inventors have made assiduous studies to obtain a photopolymerizable composition which can form a resist film excellent in adhesion to a substrate, heat resistance, chemical resistance, impact resistance and solder plating property and also use a liquid free from safety or sanitation problems as a developer.
As a result, a photopolymerizable composition has been found which allows formation of a resist film having the aforesaid intended performance on a substrate such as a printed circuit board or the like, which can use a dilute acid aqueous solution or water that ensures high safety, as a developer and which is good in storage stability. This finding has led to completion of this invention.